The stock 318 manifolds have a 1 5/8 exit hole compared to the 340/360 magnum which have an exit hole of just over 2 1/4". That's a big difference!!! That's more than a 30 percent increase in area. A 30 percent increase in area will absolutely give you a marked increase in flow.
Just look at this pic of a 318 manifold next to one of my "unmachined" 360 manifolds. You could almost stick the 318 manifold inside the 360 hole!! The second pic is one of a "machined" 360 exit hole....no comparison to the 318 manifold!
Last pic is the 318.
